Microwave wattmeter unit



July 29, 1947.

H. E. WEBBER MICROWAVE WATTMETER UNIT Filed, Dec. 13, 1943 ll llllllllllllllllllllllllllllllllllllllllllllllllllllll r:

I i I INVENTOR E. [Me-535R Patented July 29, 1941 oer-ice MICROWAVEWATTMETER UNIT Hugh E. Webber, Williston Park, N. Y., asslgnor to SperryGyroscope Company, Inc., a corporation of New York Application December1:, 194:, Serial No. 514,109

' 8 Claims.

Another object of the invention is to provide a a mounting arrangementfor connecting the hot wire element at the closed end of a coaxialtransmission line.

Still another object is to provide a by-pass condenser arrangement forelectrically closing the end of a coaxial transmission line.

Other and further objects and advantages will become apparent as thedescription proceeds.

In carrying out the invention in its preferred form, a hot wire element,comprising a filament in an evacuated bulb is mounted within the outerconductor of the coaxial transmission line'near one end of the line, acylinder or cup is mounted in the bulb at one end thereof to serve asthe inner plate of a condenser, and the filament is connected betweenthe cup and a pinterminal coaxially extending from the hot wire bulb.The 7 dimensions of the bulb are such as to conform very closely to theouter surface of the cup forming the inner condenser plate, and thedimensions of the outer cylinder of the coaxial line are such that thereis relatively little clearance between the portion of the bulbsurrounding the inner condenser plate, so that the inner surface of theouter conductor of the coaxial line serves as an outer by-pass condenserplate. The filament and the pin terminal, extending from the bulb, forma continuation of the internal conductor of the coaxial line so that thefilament is at the current anti-node or current loop of a standing waveset up in the coaxial line by the admission of high frequencyalternating current power thereto. The arrangement thus permits the highfrequency power to be supplied to one end of the line and a directcurrent connection to be made at the other end without setting up anymicrowave reflections.

A better understanding of the invention will be afforded by thefollowing description considered in conjunction with the accompanyingdrawing in which:

Fig. 1 is a side view, largely in section, of a hot 2 wire holder of aradio frequency wattmeter, showing portions or the apparatus cut by alongitudinal plane;

Fig. 2 is a sectional view of a hot wire unit as cut by a longitudinalplane through its axis;

Fig. 3 is alongitudinal sectional view correspending to Fig. 2,fragmentarily showing a hot wire holder and showing the manner in whichthe hot wire unit of Fig. 2 is mounted;

Fig. 4 is a circuit diagram of a bridge circuit in which the apparatus01' Figs. 1 and 3 may be employed.

Like reference characters are utilized through the drawing to designatelike parts.

Radio-frequency wattmeters have been proposed in which both alternatingcurrent and direct current are supplied to a hot wire or a filamentaryelement having a substantial change of resistance with temperature suchas a Wollaston wire or a barreter wire, for example. The total powerdissipation of the wire is kept constant by maintaining its resistanceconstant, variations in the alternating current power being ascertainedby measurement of the variations in direct current powertrequired tomaintain constancy of power dissipation and resistance.

Fig. 4 illustrates a suitable circuit for measuring and maintaining theconstancy of hot wire resistance. The bridge illustrated in Fig. 4comprises four arms li, l2, l3 and I connected in series parallel to asource of direct current l5, having a switch it and a current adjustingrheostat I1 for controlling the supply of direct current power to thebridge. The bridge includes the usual diagonal arm, comprising a nullindicating galvanometer I8, and a sensitivity adjusting rheostat isconnected in series between con- Jugate points 2| and 22 of the bridge.Two of the bridge arms II and I2 comprise resistors oi! substantiallyconstant or fixed resistance. The bridge arm 13 includes a resistor 23and a current-responsive instrument such as a milliammeter 24, forexample, in series, the electrical dimensions being so chosen that thetotal resistance of the'elements 2i and 24 forming the arm l3 balancesthe resistances of the arms H, I! and H to produce a null deflection inthe galvanometer II when the arm I4 is held at a predeterminedresistance value.

The arm It comprises a hot wire element adapted to have both alternatingand direct current applied to it and including a filamentary resistanceelement 25 which constitutes the actual resistance forming the bridgearm I4.

In order to obtain adequate sensitivity to var- 'ment will be highlyresponsive to variations in energy dissipated in the filament, and alsofor the purpose of protecting the filament against destruction andagainst erosion causing variations in resistance thereof, the filament25 is maintained in a vacuum or inert atmosphere and enclosed in agas-tight envelope, such as a tubular glass bulb 26.

The hot wire unit I4 is mounted in a coaxial transmission line with thehot wire 25 forming a portion of the inner conductor of the transmissionline in order that microwave energy may be supplied thereto. Forexample, a illustrated in Fig. 1, a coaxial transmission line wireholder may be provided comprising a main line 21 and a tuning stub 28.

The main line 21 comprises an outer conductor 29 in the form of a hollowcylinder, an inner conductor, comprising a rod 3| coaxial with thehollow cylinder 29 and extending from the lefthand .or microwave powerinput end 32 of the line 21 approximately to the junction with the stubline 28, and the hot wire unit l4. The latter comprises a lefthand axialpin terminal 33, the filament or hot wire 25 per se, and a righthandaxially extending pin terminal 34. In the form of apparatus illustratedin Fig. 1 a by-pass condenser 35 is formed at the righthand or directcurrent input end 36 of the line 21 in order to close the end 36 of theline with respect to microwave alternating currents and to insulate theterminal 35 of the wire 25 from the outer conductor 29.

As shown in Fig. 1 the by-pass condenser 35 comprises a substantiallycylindrical, somewhat tapered, metallic fitting 31 in contact with thepin terminal 34, surrounded by a sheet of insulating material 38 whichis surrounded, in turn, by an inner surface of a metallic fitting 39mating the fitting 31 and making direct contact with the outer hollowcylindrical conductor 29. Connections to a direct current circuit maythen be made through metallic fittings 35 and 39.

For completing a direct. current circuit from the lefthand pin terminal33 of the wire 25, to the outer conductor 29, the stub line 28 isprovided, comprising an outer hollow cylindrical conductor 4| laterallyextending from the main line 2'! and mechanically and electricallyattached to the outer hollow clylindrical conductor 29 with an innerconductor 42 in the form of a rod connected between the rod 3| and ashorting plug 43. The shorting plug 43 may be adjustable in position foradjusting the electrical length of the stub line 28, for example, bythreaded connections between the outer surface of the plug 33 and theinner surface of the hollow cylindrical conductor 4|.

For providing a sliding electrical contact between the plug 33 and therod 42, spring contact fingers 44 may be provided.

Owing to the fact that the length of line 21 in terms of wavelengthswill vary with variation in the frequency of microwave energy input atthe lefthand end 32 of the line, it may be desirable to provide meansfor matching the impedance of the input circuit (not shown) to the line21..

For example, an impedance ratio adjusting element such as a sleeve 45,slidable along the rod 3|, may be provided.

In order to avoid the necessity for tuning ad- 4 Justments'orimpedance-matching arrangements, such as the device 45, and to make theradio frequency wattmeter broadband in characteristic, that is,accurately responsive to a relatively wide range of input frequencies, Iprefer to mount the hot wire 25 as close as possible to the position ofthe current anti-node or loop in the standing wave pattern in the line21. Preferably the wire 25 is mounted directly at such a currentantinode. To this end the hot wire element I4 is so constructed that thewire 25 may have its righthand end 46 connected directly to one plate orterminal of the radio frequency by-passing condenser, and the innerplate of the by-pass condenser is mounted inside the bulb 26 of the hotwire unit l4. To connect the wire 25, it end 46 may be inserted throughan aperture in the base 48 and secured by a plate 48 spot-welded to thebase 48 within the cup 41.

For example, as illustrated in Fig. 2, a hollow cylinder, closed at oneend to form a cup 41, may

be mounted within the bulb 26, the wire 25 being connected bet-ween theclosed end or base 48 of the cup 41, and the pin terminal 33 whichextends axially to the left and makes contact with the inner conductorrod 3|. As shown in Fig. 2, the actual connection may be made through alead-in or supporting rod 49. In the arrangement shown in Fig. 2 anelectrical connection from .the cup 41 to the righthand pin terminal 34is made through a spider 5| which serve also for supporting the cup 41within the bulb 26. Preferably the bulb 26 is of but slightly greaterinternal diameter than the outer diameter of the cup 41 in order tomaintain a very small gap between the outer cylindrical surface of thecup 41 and the outer surface of the bulb 26.

In order to complete the by-pass condenser,

the inner plate of which is formed by the cup 41,

the hollow cylinder 29 may be reduced in internal diameter at therighthand end 36 to conform closely to the outer diameter of the bulb 26as shown in Fig. 3.

For this purpose, a separate reducer 52 may be provided which servesalso as a direct current connection. The reducer 52 is composed ofsuitable conducting material electrically and mechanically connected tothe inner surface of the hollow cylinder 29, and may be threaded at therighthand end 53 to receive a direct current outer terminal 54. The hotwire pin terminal 34 may be extended through a bushing 55 to serve as adirect current inner or pin terminal. Direct current leads 56 and 51 maybe made to the terminals 54 and 34 respectively. In the arrangementillustrated in Figs. 2 and 3, the coaxial line 2'! is terminated at therighthand end by the bypass condenser formed between the confrontingsurfaces of the elements 41 and 52, the actual length of these surfacestogether with their radius being sufiicient to form a condenser ofnegligible impedance with respect to radio frequency. Since therighthand end 46 of the wire 25 is in direct contact with the innerplate 4! of the by-pass condenser, the wire 25 is in a current loop andits impedance causes no reflections from the righthand end 36 of thetransmission line 21. Accordingly, an impedance-matching device slidingsleeve 45 (shown in Fig. 1) is not required and all of the radiofrequency power applied to the lefthand end input end is absorbed in thehot wire 25.

A direct current circuit is completed through the hot wire 25 from thelead 56 through the terminal 54, the reducer 52, the outer hollow cyl-28, plug 43, the contact fingers 44, the stub line inner conductor, theconducting rod 3|, the pin terminal 33 (referring to Figs. 2 and 3), theconductor 49, the hot wire 25, the cup base 48, cup 41, the spider 5|and the pin terminal 34, back to the lead 51.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a radio frequency wattmeter, a coaxial line adapted to receiveradio frequency power at one end and to be connected to a direct currentcircuit at the other end, said line having an outer hollow cylindricalconductor extending the length of the coaxial line and an inner rodextending from one end of the outer conductor partially toward theopposite end, and a hot wire unit extending from the inner end of theinner rod to the said opposite end of the outer conductor, said hot wireunit comprising a tubular bulb, a cylindrical condenser plate withinsaid bulb, a powerabsorbing wire or filament connected directly to saidcondenser cylinder atone end, and having a connection from the other endto the inner end of said inner rod of the coaxial line, the bulbdiameter conforming closely to the diameter of the condenser platecylinder, and the internal diameter of the outer hollow cylindricalconductor at the said opposite end conforming closely to the outerdiameter of the tubular bulb, whereby a bypass condenser is formed atthe end of the powerabsorbing wire. I

2. A power absorbing hot wire unit for a radio frequency wattmeter,comprising a tubular bulb, a condenser cylinder mounted therein havingan outer diameter conforming closely to the tubular bulb, alongitudinally extending pin terminal, and a power-absorbing filamentwithin the bulb connected between said pin terminal and the saidcondenser plate cylinder.

3. Apparatus as in claim 2 wherein the condenser plate cylindercomprises a hollow cup and a second pin terminal is provided whichextends axially from the opposite end of the tubular bulb and carries aspider supporting the condenser cup.

4. In a radio frequency wattmeter, a coaxial line unit comprising ahollow cylinder, 8. reducer in one end thereof, a hot wire unit mountedwithin the reducer, and an inner conductor extending within the outerconductor axially from said hot wire unit, said hot wire unit comprisinga tubular bulb conforming closely in outer surface to the inner surfaceof the reducer, a. condenser cylinder within said bulb, conformingclosely to the adjacent surface of said bulb, a power-absorbing wire orfilament connected between said condenser cylinder and said coaxial lineinner conductor, and means for making direct current connections betweeninder and said reducer.

5. A power-absorbing hot-wire unit for a radio frequency wattmetercomprising a bulb, a condenser plate mounted therein, a pair ofterminals, and a power-absorbing filament within the bulb connectedbetween one of the said terminals and said condenser plate, saidcondenser plate being connected between said filament and the remainingterminal.

6. In a power-absorbing unit connectable to a concentric transmissionline,. a by-pass condenser said condenser cylcomprising a bulb adaptedto fit within the outer conductor of the transmission line, a condenserplate contained'within said bulb and arranged to cooperate with theouter conductor of the transmission line to form a by-pass condenser,and means extending through said bulb for connecting said condenserplate to the inner conductor of the transmission line.

7. A power-absorbing hot-wire unit for a radio frequency wattmetercomprising a bulb adapted to fit within the outer conductor of aconcentric transmission line, a pair of terminals, a powerabsorbingfilament within said bulb connected to one 01' said terminals forconnection to the inner conductor of the transmission line, and acondenser plate arranged within said bulb to cooperate with said outerconductor to form a bypass condenser, said condenser plate beingconnected between said filament and the other of said terminals.

8. A power absorbing hot wire unit comprising an evacuated bulb, a firstcondenser plate therein, a second condenser plate external to the bulb,and conductor means connecting said plates respectively to oppositesides of a potential source, said conductor means including a powerabsorbing filament contained within said bulb.

- HUGH E. WEBBER.

REFERENCES CITED The following references are of record in the tile ofthis patent:

UNITED STATES PATENTS Number Name Date 2,314,764 Brown Mar. 23, 19432,081,044 Runaldue May 18, 1937 1,480,227 Stevenson Jan. 8, 19241,671,478 Marbury May 29, 1928 1,775,266 Bailey Sept. 9, 1930 1,597,835Shrader Aug. 31, 1926 2,097,100 Linsell Oct. 26, 1937 1,517,466 Schalleret a1 Dec. 2, 1924 2,270,949 Hulster Jan. 27, 1942 2,097,519 Gabriel eta1 Nov. 2, 1937 2,342,254 Dallenbach Feb. 22, 1944 FOREIGN PATENTSNumber Country Date 289,779 Germany Jan. 18, 1916' 350.170 Germany Mar.14, 1922

